The result is the same in both cases — you end up eating a good meal! `Cook up’ suggests that the various dishes were prepared rather quickly, and at short notice. The expression is considered to be informal and has the same meaning as `rustle up’.
* My grandmother managed to cook up a decent meal for the guests who arrived unannounced. * My appointment has been cancelled. Think you’ll be able to rustle up something for lunch?
MY NOTE: ‘Cooked up’ or ‘cook something up’ or ‘cook up something’ is used in informal context to mean to fabricate something. In other words, to invent something ( often dishonest or illegal) to produce a result you want.
Example: Mr.X denied the allegations of corruption in the property deal and claimed it’s a cooked up story to malign his image.
Information Courtesy: The Free Dictionary